This invention relates to a demagnetizing circuit, and particularly to a circuit construction of a demagnetizing circuit.
Conventionally, among demagnetizing circuits built into shadow mask type cathode ray tubes of color television receivers and the like, there have been those of the kind disclosed in Japanese Utility Model Publication No. S.52-28112 and Japanese Utility Model Publication No. S.52-28113. These demagnetizing circuits have the kind of circuit construction shown in simplified form in FIG. 7. The conventional demagnetizing circuit shown in FIG. 7 comprises a positive temperature characteristic thermistor (hereinafter called a PTC) 3 and a demagnetizing coil 4 connected in series with an alternating current power supply 2 by way of a power supply switch 1. A thermally actuated changeover switch 5 such as a bimetal switch opened/closed by heat produced by the PTC 3 is connected in parallel with the demagnetizing coil 4.
In a modified version of this circuit construction, the thermally actuated changeover switch 5 is connected in series between the PTC 3 and the demagnetizing coil 4 and a bypass resistor 6 is connected in parallel with the demagnetizing coil 4 as shown in FIG. 8.
In the conventional circuit construction shown in FIG. 7, when the power supply switch 1 is closed, current from the power supply 2 flows through the PTC 3 and the demagnetizing coil 4, demagnetizing a shadow mask plate of a cathode ray tube, and the PTC 3 produces heat. As time passes the resistance of the PTC 3 rapidly rises and the current flowing through the demagnetizing coil 4 falls, and the current flowing through the demagnetizing coil 4 is thereby controlled according to the resistance-temperature characteristic of the PTC 3. Also, heat produced by the PTC 3 is transmitted to the changeover switch 5, which had been open. When the changeover switch 5 is heated by the PTC 3, it consequently closes and as a result current from the power supply 2 flows through the changeover switch 5 and the flow of current through the demagnetizing coil 4 is cut off.
In the modified example shown in FIG. 8, on the other hand, the changeover switch 5, which is connected in series between the PTC 3 and the demagnetizing coil 4 and had been closed, is opened by heat produced by the PTC 3. Current from the power supply 2 consequently flows through the bypass resistor 6 and as a result, as in the example shown in FIG. 7, the current flowing from the power supply 2 to the demagnetizing coil 4 is cut off.
Demagnetizing circuits having the circuit constructions of the conventional examples of FIGS. 7 and 8 have the merits that it is possible to automatically cut off residual demagnetizing current that is still flowing after the demagnetization is complete, and to eliminate both picture shake of a cathode ray tube caused by effects of residual demagnetizing current, and picture disturbance caused by intruding noise from the power supply 2.
However, in recent years, due to their connection with the impediment by the VDT and the like, a strong demand has arisen to suppress low-frequency electromagnetic fields produced by cathode ray tubes, and the demagnetizing coil 4 in the demagnetizing circuit has been seen as one source of such low-frequency electromagnetic fields. However, in demagnetizing circuits having the conventional constructions described above, because one side of the power supply 2 and one side of the demagnetizing coil 4 are connected even after the changeover switch 5 operates, there has been the problem that it has been extremely difficult to suppress the production of a low-frequency electromagnetic field.